Unfreezable Defects

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A EUROfusion-funded research paper recently appeared in the high-impact journal Nature Materials. ‘Quantum de-trapping and transport of heavy defects in tungsten’ (Arakawa, K., Marinica, M., Fitzgerald, S. et al.). It provides new information, significant for the assessment of structural and functional integrity of plasma-facing and superconducting materials. The new study reveals how defects move in tungsten, and offers insight into what can be happening under irradiation in materials at cryogenic temperatures, for example in superconducting magnets used in magnetically confined fusion (MCF) devices like tokamaks (JT-60SA in Japan and the future ITER device in France) and stellarators (Wendelstein 7-X in Germany).

A Scientific Discovery

Fusion research and European efforts to realise fusion-generated electricity face two principal scientific challenges: plasma stability and understanding the effect of exposing materials to fusion neutrons.

“We sought to examine what happens to defects, created by neutron irradiation in heavy materials like tungsten at low temperature, to provide insight into the fundamentals of defect dynamics and also into the processes occurring in superconducting magnets at cryogenic temperatures under irradiation,” says a co-author Sergei Dudarev, Head of the Materials Modelling Group at the United Kingdom Atomic Energy Authority. “The theory of the effect was developed in 2017 by Tom Swinburne, who was then a EUROfusion Fellow, and his colleagues. And now the effect has been directly observed experimentally.”

What the team observed was that defects continue to move. At the atomic level, the defects constantly jump over microscopically small distances, and these jumps were directly observed in a microscope at various temperatures, to see whether they would stop at temperatures close to absolute zero. They did not.

Micro-Collaboration

The paper is a result of a collaboration, and even friendship, built between 18 scientists from several institutions in Japan, France, and the UK involved in fusion materials research. The study has been led by Kazuto Arakawa, a professor at Shimane University in Japan, who specializes in microstructural analyses using transmission electron microscopy and is an authority in his field.

“Japan has long been known for its expertise in microscopy,” shares Sergei. Through scientific collaboration and relationships built over the past decade, scientists from France and the UK drew upon their Japanese counterparts’ expertise in microscopy.

EUROfusion funding, as well as in-kind contributions, where crucial to carry out the work that resulted in this paper. Together, the scientists tested their theories, using electron microscopes to see and track defects in materials near absolute zero, and even video-recording them moving.

Kazuto Arakawa is a professor at Shimane University in Japan who specializes in microstructural analyses using transmission electron microscopy and is an authority in his field.
Picture: courtesy of Professor Kazuto Arakawa

Click to see the researcher's VIDEO of one-dimensional motion of nanoscale SIA (self-interstitial atom) clusters.

Breaking the Arrhenius Law

A century-old theory, Arrhenius' law, has been known to explain what seems to be a common sense behaviour: that defects in materials at absolute zero (−273 °C / −460 °F) remain frozen. They do not move. Long held to be true, this law had only been directly verified close to room temperature or above.

Arakawa and the team of scientists not only came up with a theory that challenged Arrhenius’ law, they produced video evidence proving it wrong. Using electron microscopy, they observed that heavy defects generated by irradiation in cryogenically-frozen tungsten move due to quantum fluctuations in the crystal lattice. This diffusion of heavy defects is driven by the Planck constant and the quantum Heisenberg uncertainty principle, stating that atoms in a crystal lattice never come to rest and continue moving even at absolute zero temperature.

Certainty Welcomed

For the still new area of fusion technology which faces great unknowns, any new certainty is most welcome! Arakawa and his team’s findings about the fundamental subtleties of motion of defects will help fusion scientists and engineers predict with certainty how materials handle neutron irradiation when fusion occurs in deuterium-tritium (D-T) plasmas. Based on the present understanding, D-T plasmas promise to be the only plasmas capable of generating more energy in the form of heat than the energy required to trigger and control fusion in the plasma.

Now it is beyond a doubt that defects move at super low temperatures; you cannot freeze them out. That is a surprise, one that will impact the development of materials for extreme environments, and that also might help with the development of future superconductors.

What is a defect and why this is relevant to fusion research?

Defects are imperfections in the arrangement or stacking of atoms that make up a given amount of a material. A defect-free material has specific properties: it responds in a predictable way to heat, stress, impact and so forth. Defects have different properties. The more defects there are in a given material, the harder it is to predict how it is going to behave.

Acknowledgement

This work has been carried out within the framework of the EUROfusion consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.